1. Field of the Art
This invention relates to a focus demand for use on a television camera or the like.
2. Description of the Prior Art
In television cameras, the focusing lens is usually driven through a servo means including a drive member like a motor and a control mechanism which controls the operation of the drive member. The servo means is associated with a focus demand which supplies a control signal to a control section of the servo means. Generally, the focus demand is largely constituted by a focus operating means in the form of a handle or the like which is manually operated by an operator, and a sensor means like a potentiometer which is arranged to detect the extent of the manual operating efforts exerted on the focus operating means and to produce an electric signal proportional to the extent of the operating efforts. The output signal of the sensor means is applied as a servo control signal to the control section of the servo means to operate the drive member according to the servo control signal for a focusing operation in a range from the nearest subject distance to infinite.
In this connection, the displacement of the focusing lens is not in linear relation with the subject distance, and varies in the fashion of an exponential function. Accordingly, if the focusing lens is moved in proportion to the extent of manual efforts on the focus operating means, the sensitivity of the focusing operation becomes overly high toward the infinite end of the focusing range to make delicate or fine focusing operations difficult. Conversely, the response of the focusing lens to the manipulation of the focus operating means becomes lower toward the nearest subject distance position. The focus maneuverability at the infinite position can be improved by broadening the operating stroke range of the focus operating means. On the other hand, the response of the focusing lens to the focus operating means can be improved by narrowing the operating stroke range of the latter. However, a broadened operating stroke range of the focus operating means will further deteriorate the response of the focusing lens at or in the vicinity of the nearest subject distance. On the contrary, a narrowed operating stroke range will further increase the sensitivity at or in the vicinity of the infinite position to deteriorate the maneuverability of the focusing lens to a considerable degree. Namely, neither of these measures can improve the focus maneuverability over the entire focusing range from the nearest subject distance to infinite.
In an attempt to overcome these problems in focusing operations, there have been developed mechanisms which are designed to switch the stroke range of the focus operating means in such a manner as to broaden the stroke range in focusing operations at or near the infinite position and to narrow the stroke range in operations at or in the vicinity of the nearest subject distances position, as described in Japanese Laid-Open Patent Application 2-154212.
In this manner, the focus maneuverability by the focus control device can be improved to a certain degree by changing the drive rate per unit stroke length of the focus operating means according to the subject distance. However, it is often found troublesome and annoying for an operator to switch the drive rate while operating a camera. Besides, even if the effective stroke range of the focus operating means is varied stepwise to provide two or three different focus drive rates, it is still difficult to improve the focus maneuverability to a sufficient degree over the entire focusing range from the nearest subject distance to the infinite position, thus limiting the overall improvement of the focus maneuverability of the focus control device.